Coating of electrically conductive surfaces

ABSTRACT

MONOLAYERS OF MONODISPERSE SYNTHETIC PARTICLES ARE DEPOSITED FROM AN AQUEOUS SUSPENSION EMPLOYING AN ELECTRICAL FIELD OF BETWEEN ABOUT 1.5 AND 3 VOLTS PER CENTIMETER.

United States Patent 01 ice US. Cl. 204181 6 Claims ABSTRACT OF THE DISCLOSURE Monolayers of monodisperse synthetic resinous particles are deposited from an aqueous suspension employing an electrical field of between about 1.5 and 3 volts per centimeter.

This invention relates to coating of electrically conductive surfaces such as metal with monodisperse plastic spheres, and more particularly relates to a method of ob taining a monolayer.

Electrical deposition of synthetic resinous particles from an aqueous dispersion on a conductive surface is well known. However, in accordance with the prior art, coat ings of substantial thickness are built up and such coatings generally are from polydisperse polymer latexes or dispersions. Thin layers of monodisperse particles are known and optical characteristics are desirable for many purposes. The desirability of such monolayers is discussed in the Journal of the Optical Society of America, vol. 44, No. 8, 603-609, August 1954, in a paper by T. Alfrey, Jr., E. B. Bradford and I. W. Vanderhofi'. The technique employed for obtaining monolayers of a monodisperse latex is to evaporate a liquid which is employed as a dispersing agent for a monodisperse latex.

It would be desirable if there were available a rapid method for the preparation of a monodisperse layer of plastic particles on an electrically conductive substrate.

It would also be desirable if there were available a method for the preparation of a monolayer of monodisperse particles on a substrate.

It would also be desirable if there were available a method of preparing a monolayer of monodisperse particles on a substrate wherein soluble contaminants such as dispersing agents could readily be removed therefrom.

These benefits and other advantages in accordance with the present invention are achieved in a method of obtaining a monolayer on an electrically conductive substrate, the steps of the method comprising providing an electri cally conductive substrate, providing an electrically conductive liquid dispersion of monodisperse synthetic resin ous particles of colloidal size, passing an electric current through the dispersion and electrically conductive sub strate under an electrical field to l to 4 volts per centimeter for a time sufficient to provide a monolayer of particles thereon.

Monodisperse uniform particle size latexes which are useful in the practice of the present invention are well known and are discussed in the Alfrey, et al. article hereinbefore set forth. Beneficially, such latexes are useful at concentrations up to 50 percent solids. Advantageously for the practice of the present invention, dispersions are diluted to a concentration of from about 0.1 percent to about 5 percent, and advantageously to a concentration of from about 0.5 percent to 2 percent, such percentages being percentage by weight. Such monodisperse latexes generally have an ionic stabilizing agent such as a sodium salt of a sulphonated organic compound such as an alkyl benzene or other anionic surfactant. Such latexes may also be stabilized with cationic surfactants.

A wide variety of conductive substrates may be em- 3,598,709 Patented Aug. 10, 1971 ployed. Electrically conductive metals are eminently satisfactory as well as water resistant conductive non-metallic coatings such as those obtained by the mixture of conductive carbon particles with a synthetic resinous binder. Metals such as aluminum, steel, copper, tin, lead, stainless steel, platinum and the like are beneficially employed.

In depositing monolayer coatings in accordance with the present invention, direct current or pulsed direct current is used. The material to be coated forms one electrode. The remaining electrode may be any convenient generally non-reactive material; that is, generally non-reactive under the conditions of deposition, such as copper, lead, carbon, graphite and the like. In cases where the dispersion has been stabilized with an anionic surfactant, the workpiece or substrate to be coated is the positive electrode, whereas, when the cationic surfactant is employed the workpiece is the negative electrode.

It is critical to the practice of the present invention that the electric field be maintained within the range of from about 1 to 4 volts per centimeter, and beneficially, from about 1.5 to about 2.5 volts per centimeter. If the voltage is lower than about one volt per centimeter, n0 deposition occurs, and if the voltage is greater than 4 volts per centimeter, multilayer deposition tends to occur. For most applications, it is particularly advanta geous to operate within the range of 1.5 to 2.5 volts per centimeter.

By way of further illustration, the inside bottom surface of a cylindrical stainless steel beaker is coated with a layer of sealing wax about 5 millimeters in thickness. A 3.7 centimeter outside diameter glass tube about 7.7 centimeters in length is wrapped with a sheet of aluminum foil, the foil held in place with pressure sensitive tape. The glass tube is positioned within the beaker to provide uniform spacing from the inner wall of the beaker of one centimeter. A monodisperse polystyrene latex with a particle diameter of 109910.009 micron and stabilized with an anionic surfactant is diluted to one weight percent solids with water and placed in the stainless steel beaker. A direct current power source delivering 2.0 volts is connected to the stainless steel beaker (negative electrode) and to the aluminum foil (positive electrode). Voltage is applied for a period of seconds and the aluminum electrode removed. The aluminum foil is covered with a monolayer of polystyrene latex particles and is washed by immersion in fresh portions of distilled Water. The foil is dried in air and the weight gain is determined to be 6 milligrams. The calculated weight for a uniform monolayer requires 7 milligrams. The foil is examined under a microscope and a monolayer of particles is observed. No indication is found of the buildup of a second layer of particles. The particles adhere to the aluminum with sufficient strength to permit handling. A portion of the foil is employed for calibration under a light microscope wherein minor amounts of the monolayer are dislodged by gentle scraping to provide particles of a predetermined size. Similar results are obtained when the aluminum foil is replaced with platinum, copper and tin.

Deposition of latex particles to a monolayer must be done in such a manner as to avoid buildup of a second layer of particles before the first or monolayer is complete. This is most easily accomplished by calibration of the deposition equipment, by providing an electrode of known weight upon which the particles are to be deposited, depositing substantially less than a monolayer of particles on the surface, washing and weighing the particles and calculating the percentage of surface covered. As the deposition of particles in accordance with the practice of the present invention is proportional to the square root of the time during which the electric field is applied, the length of time is readily calculated for the deposition of a monolayer. As the applied electrical field is increased,

the rate of deposition increases rapidly and control is more difiicult. When voltages as high as volts are employed, the deposition of particles is generally directly proportional to time, and evidence of second layer formation is apparent. The particles on being deposited on the metallic or conductive substrate adhere sufficiently well that the coated electrode may be washed by immersing in a suitable washing solution. Thus, the particles can be freed from at least a major portion of the stabilizing agent or agents originally present. When the substrates are dried; that is, solvents and/0r water removed therefrom, they may be handled with care without dis lodging particles. Such coated substrates are particularly advantageous in that they provide a convenient source of dry and relatively contaminant-free particles of known size. Small quantities such as are desired for microscopic calibration are readily dislodged by scraping the particles from the electrode with a spatula or fine brush.

Advantageously, such particles are also employed with benefit for spacing where it is desired to maintain two surfaces at a generally fixed distance, for example, in spacing a microscope cover glass for a slide. A reflection refraction grating and the like are readily prepared employing as a substrate a polished metal surface. For most purposes, it is desirable that the polymeric particles deposited on the substrate be of a rigid nature; that is, have a suflicient strength that on evaporation of water they remain relatively undistorted. The desirable lateXes generally are those which are not film forming. However, where it is desired to form a film, the so-called non-film forming latex particles may be heated to a temperature sufficiently high to become thermoplastic and form a continuous film. By the term non-film forming is meant that the latexes on being coated on a substrate, the water evaporating at room temperature, do not form a coherent film. Generally for rigid particles, polystyrene is eminently satisfactory unless the particle are to be employed at higher temperatures. If higher temperature service is required, such latexes may be provided with suflicient crosslinking that the particles are not thermoplastic.

Deposition of particles is primarily a physical phenomenon and is not dependent upon the composition of the polymer of the latex. The selection of the particular polymer is primarily dependent upon the end use to which the coated substrate is intended. For most purposes, polystyrene is particularly desirable as the polymer density 4 approximates that of water and very stable dispersions of large particles are available.

As is apparent from the foregoing specification, the present invention is susceptible of being embodied with various alterations and modifications which may differ particularly from those that have been described in the preceding specification and description. For this reason, it is to be fully understood that all of the foregoing is intended to be merely illustrative and is not to be construed or interpreted as being restrictive or otherwise limiting of the present invention.

What is claimed is:

1. A method of depositing a monolayer of monodisperse polystyrene particles, the method comprising providing an electrically conductive substrate and an electrically conductive liquid dispersion of monodisperse polystyrene particles of colloidal size,

passing an electrical current through the dispersion and electrically conductive substrate under an electrical field of from about 1 to 4 volts per centimeter for a time sufiicient to provide a monolayer of said particles thereon.

2 The method of claim 1 wherein the electrical field is from about 1.5 to about 2.5 volts per centimeter.

3. The method of claim 1 wherein the conductive substrate is a metal.

4. The method of claim 1 wherein the dispersion is from about 0.1 percent to about 5 percent by weight of polystyrene particles.

5. The method of claim 4 wherein the dispersion is from about 0.5 percent to about 2 percent by weight of polystyrene particles.

6. The method of claim 1 including the step of removing the conductive substrate from the dispersion and washing the coated substrate with a non-solvent for the particles.

References Cited UNITED STATES PATENTS 2,848,391 8/1958 Fahnoe et al. 204181X 2,851,408 9/1958 Cerulli 204181 2,878,140 3/1959 Barr 204l81X 3,304,250 2/ 1967 Gilchrist 204181 3,305,467 2/1967 Igras et al. 204l81 PATRICK P. GARVIN, Primary Examiner W. J. SHINE, Assistant Examiner 

